In the operation of tractor-trailers, one of the most serious and destructive incidents is the inadvertent excessive swinging of the trailer from the centerline of travel so that the tractor and trailer close upon each other similar to the blade of a jackknife. This action, referred to as "jackknifing" is probably the most feared occurrence by all operators in the trucking industry. Jackknifing commonly occurs on slippery roads due to ice, rain or the like particularly on an incline. In a normal running of a tractor-trailer rig on a straight stretch of road, the fifth wheel pivots very little, if any. When making a turn, the fifth wheel pivots slightly and a deviation of up to ten degrees from the centerline of travel is more than adequate for highway travel. However, under slippery road conditions, this angle of deviation, regardless of its extent, will become a "critical angle" when the tractor-trailer rig begins to skid and swerve. At this time, a brief opportunity exists for the driver to correct the steering in an attempt to realign the tractor in the direction of the trailer as long as the "critical angle" is small enough to permit this. If the "critical angle" becomes too large, the moving mass of the trailer overcomes the ability to correct the tractor quickly enough and the trailer will actually whip the tractor around the fifth wheel pivot point smashing the tractor into the trailer. Various devices have been proposed to reduce jackknifing such as by physically constraining the pivotal movement of the tractor from passing through a predetermined angle. One such device is shown in U.S. Pat. No. 2,959,429 issued to W. D. Lavelle on Nov. 8, 1960 and entitled Fifth Wheel Safety Device. In the Lavelle patent, retractable rams are provided on the fifth wheel which may be moved into arcuate slots on the trailer body so that when an excessive turning angle is reached the rams engage the terminal ends of the slots. While the Lavelle device does provide some protection against jackknifing, it can be understood that such a device must be of extremely strong construction since the impact forces encountered are of substantial magnitude particularly when the angle at which the pivotal movement of the tractor is constrained is quite large. Furthermore, jackknifing can occur in such a device at smaller turning angles depending on the driving abilities of the driver and if the driver is unaware of incipient danger or distracted, insufficient time is available for taking corrective action utilizing steering control to regain the tractor and trailer thereby preventing jackknifing.